1. Field of the Invention
This invention relates to thermoplastic coating powders adapted to be applied by fusion coating processes and more particularly to thermoplastic coating powders that have improved adhesion to substrates.
2. Description of the Prior Art
Fusion coating processes are those in which a powdered coating material is distributed over a substrate and is heated to cause the powders to fuse into a continuous protective or decorative film. The fluidized bed and the electrostatic spray coating processes are representative of fusion coating processes.
Fusion coating processes are attractive alternatives to solution coatings since solvents are not required. Not only is the cost of solvents saved, but, more importantly, the need for collection and recovery systems to guard against fire, explosions and contamination of the atmosphere is eliminated.
Except in those specialized circumstances in which stripable coatings are desired, the performance of a protective or decorative coating is dependent upon the tenacity with which the coating material adheres to its substrate. Adhesion is necessary to prevent the coating material from being peeled away from the substrate and to protect the substrate from the spread of underfilm corrosion should any discontinuity in the coating layer occur.
It has long been recognized that the adhesion between a coating and a substrate can be very much improved through the use of primers. In recent years primers, variously comprised of reactive thermosetting resins and thermoplastic adhesives have been specially adapted for use in connection with fusion coating processes. Unhappily, these primers are applied to the substrate by solution coating techniques and so, in addition to being a time consuming and bothersome step, some of the advantage of using the solventless fusion coating processes is lost since a solvent recovery system may be required.
To avoid having to prime a substrate, it has been proposed to mix adhesion promoting materials directly into coating powders. For example, U.S. Pat. No. 3,562,205 discloses the use of organic nitrogen compounds and metal fillers to improve the adhesion of thermoplastic coating powders to a substrate. It is known that reactive epoxy resins are useful in improving the ability of nylon to adhere to a substrate. It is also known that the adhesion of nylon coating powders is greatly improved by dry blending a reactive epoxy, a curing agent for the epoxy and a polyvinyl acetal into the powder.
While the inclusion of an epoxy resin, a curing agent, and a polyvinyl acetal as dry blend additives to nylon coating powders will provide markedly improved adhesion, coating powders based upon dry blends are not preferred. Quite generally, dry blends are not satisfactory for use in electrostatic coating processes since the separate components of a dry blend coating powder will differ in their ability to accept a charge, which may lead to disuniformities in the coating. Dry blends may prove unsatisfactory for use in fluidized bed coating processes since the separate components will be removed from the fluidized bed at different rates depending upon such factors as their size, shape, softening point, and density. For these reasons, the higher quality coating powders used in heat fusion processes are prepared by melt mixing the fillers, pigments, stabilizers, plasticizers, etc. into the powder. Most commonly this is done by processing the several raw materials in a mixing extruder, extruding small diameter rods, chopping the rods into pellets and grinding the pellets to the required size.
Attempts to melt mix an epoxy, a curing agent and a polyvinyl acetal with nylon in an extruder have failed since, at the temperature at which the nylon melts, the epoxy becomes reactive and fuses the entire system together into an intractable and useless mass. For this reason, the utility of this preferred system for improving the adhesion of a nylon coating powder has been seriously limited.
Accordingly, it is an object of this invention to improve the adhesion between a substrate and a fused nylon powder coating.
Another object of this invention is to provide melt mixed, homogenous nylon coating powders that, when fused, will adhere to a substrate.
Briefly, these and other objects of this invention are achieved by melt blending minor amounts of a reactive epoxy resin and a polyvinyl acetal into a nylon coating powder while omitting an epoxy curing agent from the formulation. It has been discovered, somewhat predictably, that most epoxies will not react at the melting point of the nylon in the absence of a curing agent for the epoxy; it has been discovered, quite surprisingly, that the omission of the curing agent does not observably reduce the adhesion of the nylon coating powders to a substrate.
The selection of an epoxy resin for use in this invention is not critical and may include epoxy resins having an epoxide equivalent weight of from about 200 to over 2,000. The particular epoxy may be selected, for example, with regard to its melt viscosity so that it may be used to raise or lower the melt viscosity of the coating powders. Quite generally, the additions of expoxies having an epoxy equivalent weight of between 200 to 2,000 and in an amount of from about 2 to 15 parts by weight per 100 parts of nylon have proved useful.
The polyvinyl acetal resins which have been used in the practice of this invention have been limited to those that can readily be obtained in the marketplace which are polyvinyl butyral and polyvinyl formal, although there is no reason to believe that others might not be effective. The polyvinyl acetals are commonly prepared by the saponification of polyvinyl acetate to yield polyvinyl alcohol which, in turn, is acetalated with formaldehyde or butyraldehyde. The resulting resins predominate in polyvinyl acetal with minor residues of polyvinyl alcohol and polyvinyl acetate in the polymeric chain. Polyvinyl acetals are available, for example, from Monsanto under their trademarks Butvar and Formvar and from Farbwerke Hoechst under its trademark Mowital. The inclusion of from about 1 to 10 parts by weight polyvinyl acetal per 100 parts nylon have proved effective.
The nylons that are found most useful in the practice of this invention as coating powders are types 11 and 12 nylon which, due to their lower melting point, make better coating materials than the more common types 6 and 6/6 nylon.